Postponing action on a rail authorization until Congress takes up broader surface transportation legislation next year may provide Amtrak supporters with a tantalizing opportunity to solve their long-term funding problem.

Authorizers will already be under pressure to overhaul long-term financing for the Highway Trust Fund next year. That could provide an opening for efforts to include a permanent funding stream for passenger rail as part of a bigger deal.

The idea isn’t completely new.

In his fiscal 2014 budget, President Barack Obama proposed creating a new Transportation Trust Fund that would incorporate funds for highway, bridge, transit and rail maintenance and projects.

Currently the Highway Trust Fund spends 80 percent of its receipts on roads and bridges, while the remaining 20 percent goes toward transit. Amtrak, meanwhile, has to fight for an annual appropriation, leaving the passenger railroad at the mercy of lawmakers who question the federal role in funding intercity passenger rail service.

The result for Amtrak has typically been appropriations just big enough for the railroad to limp along but short of what the nationalized company says it needs for major capital improvements.

Obama’s proposal would inject savings from pulling U.S. troops out of Iraq and Afghanistan into a new trust fund that would finance all modes of surface transportation.

While Obama’s proposal to use war savings has gained little challenge, many authorizers see the need to devise a long-term fix for the Highway Trust Fund.

The trust fund’s revenue from motor fuel taxes is lagging behind authorized spending, as motorists shift to more fuel-efficient vehicles and drive less. The Congressional Budget Office projects that the trust fund will have insufficient resources to meet its obligations by the end of fiscal 2014.

A makeover of the system for funding surface transportation could give Amtrak advocates a chance to fight for a share of the new money — especially if Congress punts on taking up rail authorization this fall and decides instead to wrap it into a big highway bill next year.

“A new model for investment is needed,” Joseph H. Boardman, the railroad’s president and chief executive, told Congress earlier this year. “If we do not obtain one, the outlook for the system’s capacity and condition is grim.”